Vacuum and Instrument Air Systems

A vacuum system is the air supply of an airplane's air-driven gyro instruments. In the classic light trainer an engine-driven pump draws filtered air through the cases of the attitude indicator and the heading indicator, and the stream of air spins their rotors. A suction gauge on the panel shows whether the pressure difference is large enough to keep them at speed. Some airplanes use a venturi on the outside of the airplane instead of a pump, and some push the air through the instruments with a pressure pump; together these are the instrument air systems.

The system matters out of proportion to its simplicity. When it fails, the gyros do not stop at once: they run down over several minutes, and the attitude indicator drifts into a slow, believable bank or pitch, often with no flag. Recognizing the failure, knowing which instruments it removes and flying on the rest, partial panel, appear in the EASA PPL, CPL, IR and ATPL theory syllabi and in the FAA private pilot and instrument rating tests.

Air-driven gyro instruments

An air-driven gyro is a rotor with small buckets cut into its rim. Jets of air entering the instrument case strike the buckets and spin the rotor, much as water turns a waterwheel or turbine. The instrument works through rigidity in space, which grows with the rotor's mass, with how far that mass lies from the axis and with its speed (see gyroscopic principles). Of the three, only speed changes in flight, and it depends entirely on the air supply. A rotor that slows loses rigidity and precesses more readily under bearing friction and the erection forces, so its indications become sluggish and then wrong.

ATPL texts give typical rotor speeds of about 15,000 rpm for an air-driven attitude indicator and about 22,500 rpm for an electric one. Electric gyros are heavier and more expensive, but they spin faster, their speed is controlled more precisely, and they are not affected by a dirty filter or by the reduced suction an air system may suffer at high altitude. The EASA theory syllabus asks candidates to describe both ways of driving gyroscopes, by air or vacuum and electrically, and the indications that go with each.

The usual light-airplane arrangement splits the power sources. The attitude indicator and the heading indicator (direction indicator, DI) are vacuum-driven, and the turn coordinator or turn and slip indicator is electric, so that a failure of either source leaves at least one gyro instrument working (see turn and slip indicators). FAA material notes that most airplanes have at least two sources of power for this reason, so that one source of bank information survives. Some installations reverse the arrangement, some run every gyro on air or every gyro on electricity, and glass cockpits replace the vacuum gyros with an attitude and heading reference system and an electric standby instrument.

Engine-driven vacuum pumps

In the FAA's description a typical vacuum system consists of an engine-driven vacuum pump, a relief valve, an air filter, a gauge and the tubing that connects them. The pump is a vane-type pump mounted on the engine's accessory case, and its capacity depends on the number of gyros it has to drive. The air follows a fixed path:

1. Air is drawn into the system through an air filter, which keeps foreign matter out of the instruments.
2. It flows through the attitude and heading indicators, spinning their rotors.
3. A suction relief valve, or vacuum regulator, stops the suction exceeding its prescribed limit. It does so by admitting outside air, at higher pressure, into the vacuum line through a filter of its own.
4. The pump discharges the air overboard or into other systems, such as the pneumatic de-icing boots (see de-icing boots and fluid ice protection).

Two kinds of pump are in service. A wet pump has steel vanes lubricated by engine oil, and the air it discharges carries that oil: before the air can inflate rubber de-icing boots an oil separator must remove it, and the system also has a gate check valve. A dry pump has carbon vanes and needs no engine oil supply, no separator and no gate check valve. Its weakness is that it can fail without warning. That is why many airplanes carry a standby vacuum system, dual pumps or an electric backup attitude indicator; the typical light-airplane electrical system drawn in the FAA's handbook includes a switch and circuit breaker for a standby vacuum pump and a feed to a low-vacuum warning light.

A twin can drive a pump from each engine. An example in the FAA's maintenance handbook has two engine-driven pumps, two relief valves and two flapper-type check valves on a common vacuum manifold. A check valve lets air flow one way only, so a pump that stops cannot let air back into the manifold, and the other pump goes on supplying the instruments.

Some airplanes push the air through the gyros instead of drawing it. In a pressure system the pump's outlet supplies the filtered air, and the FAA quotes the same working range, usually 4.5 to 5.5 inHg, for vacuum and pressure systems alike. ATPL texts add that the pressure difference an air system can produce falls as the outside pressure falls, so at high altitude the rotors may run slow; a contaminated filter has the same effect at any height.

Venturi systems

A venturi tube produces instrument suction without a pump. FAA material defines it as a specially shaped tube attached to the outside of an airplane to produce suction for the gyro instruments. Air accelerates through its narrow throat and its static pressure falls, by Bernoulli's principle (see airflow and Bernoulli's principle). A line from the throat to the instrument cases lets that low pressure draw air through the gyros, and FAA maintenance material notes that the air going into the instruments is normally filtered.

A venturi has no moving parts and needs no drive from the engine. Its suction, though, comes from the airflow through it: the faster the air at the throat, the lower the pressure there. The suction therefore varies with airspeed and is weak at low speed.

Suction gauge and limits

The suction gauge shows how far the pressure in the system is below the surrounding pressure: FAA material measures vacuum in inches of mercury (inHg) less than ambient pressure. The value an installation needs varies, but it is usually between 4.5 and 5.5 inHg, and the gauge is generally marked to show the normal range. Some airplanes also have a warning light that comes on when the suction falls below the acceptable level, and some instruments show a red flag.

EASA makes that indication a requirement. Under Part-NCO an airplane flown in VMC at night, or in conditions in which it cannot be kept on its flight path without additional instruments (NCO.IDE.A.120), or under IFR (NCO.IDE.A.125), must have "a means of indicating when the supply of power to the gyroscopic instruments is not adequate". CAT.IDE.A.130 requires the equivalent, for the required flight instruments, on commercial flights at night or under IFR. In an air-driven installation the suction gauge or the low-vacuum light is that means.

Engine speed matters. The FAA notes that at idle power the vacuum gyros might not be up to operating speed and might precess more rapidly than in flight. In its instrument check the pilot listens for abnormal sounds as the gyro spools up and, while taxiing, sees that the instrument shows turns in the correct direction with normal precession.

Where pneumatic de-icing boots share the system, the suction gauge and a pneumatic pressure gauge also show whether the boots are working, with range markings for boot operation.

Failure modes and indications

The classic failure is the sudden loss of the pump. The suction gauge drops to zero, but the rotors keep spinning and run down over several minutes. The attitude indicator tilts slowly into a plausible bank or pitch, and the heading indicator wanders, often without any flag. A pilot who follows the attitude indicator banks the airplane while the turn coordinator shows a turn, the heading changes and the altimeter and vertical speed indicator show a descent. In VMC the failure is an inconvenience; in IMC it is an emergency.

A gradual failure, from a leak, a clogging filter or a worn pump, is harder to see, because the gyros do not fail but become less accurate. ATPL texts give two exam examples of a rotor running slow. A heading indicator's latitude nut applies a fixed torque, and precession for a given torque increases as rotor speed falls, so a slow rotor over-corrects the apparent drift. An air-driven turn indicator under-reads, because a slower rotor needs less spring deflection to precess at the actual rate of turn: the airplane is turning faster than the needle shows. An overspeeding rotor gives the opposite errors: the latitude nut under-corrects and the turn indicator over-reads.

The suction gauge and the pattern of the failure tell the pilot what has gone:

• low suction, attitude and heading indicators both drifting: vacuum system failure;
• normal suction, attitude indicator alone tumbled or wrong: failure of that instrument, with the heading indicator still usable;
• turn coordinator flag or a stationary needle in a turn, suction normal: electrical failure, with the vacuum gyros still working.

The pitot-static instruments need no power at all and are the common reference in every case. The defense is the habit the FAA recommends: cross-check the instruments routinely, with the suction gauge in the scan.

View full schematic

Partial-panel flying

Partial panel, or limited panel, flying is control of the airplane with one or more gyro instruments failed. After a vacuum failure in the typical trainer the scan is rebuilt around the instruments that remain (see instrument layout and scan technique):

1. Cover the failed attitude and heading indicators, so that their slowly growing errors cannot mislead.
2. Bank from the turn coordinator or turn and slip indicator: a level miniature airplane with the ball centered means wings level. The placard NO PITCH INFORMATION is a reminder that the instrument shows rate of turn and roll, not attitude; a steady altimeter does not prove the wings are level, since a balanced turn can hold altitude.
3. Heading from the magnetic compass, read only in straight, level and unaccelerated flight. Heading changes are made as timed rate-one turns of 3° per second, so 30° takes 10 seconds and 180° one minute (magnetic compass errors). On an easterly or westerly heading an acceleration makes the compass show a turn that is not happening; it should not be chased.
4. Pitch and power from the altimeter, vertical speed indicator and airspeed indicator, with known power settings.

The workload is high, so the pilot tells ATC. Under radar control the FAA's AIM describes a no-gyro vector or approach for a pilot whose directional gyro is inoperative or inaccurate: the pilot advises ATC and requests it, the controller says "turn right" and "stop turn", the pilot makes every turn at standard rate and starts it as soon as the instruction is received, and on the final approach of a surveillance or precision radar approach turns at half standard rate. If visual conditions are within reach, the pilot heads for them.

A glass cockpit fails differently. If the attitude and heading reference system fails, red crosses replace attitude and heading, while the air data remain valid; the pilot flies the standby attitude indicator and the compass. An electric standby attitude indicator gives a vacuum airplane the same protection.

Library articles are written for study and exam preparation. They do not replace your aircraft's approved documentation, your operator's procedures or the regulations themselves.